A standard differential housing is a massive hollow casting that is formed with several holes of different diameters aligned normally on a plane at 90° to each other. Internally it has other surfaces that must be machined to perfectly planar or part-spherical shapes. All of this machining must be done to a very high precision to accommodate the various shafts and gears that are eventually installed in the housing.
In the classic system for doing this, described in German patent document 41 42 121 of A. Baudermann the workpiece is moved from machining station to machining station and is clamped at each station as it is worked on there, typically by accurately boring out and grinding the holes which extend to the exterior and by milling interior surfaces, normally around the holes. Such a method is very slow and makes it quite difficult to align the various holes and surfaces perfectly with one another.
Various other methods and systems are described in WO 02/00390 of P. Modig, German patent 37 22 180 of H. Ullmann, and U.S. Pat. No. 5,781,983 of M. Gruner. All these methods involve handing the workpiece off from one holder to another and complex positioning systems for the workpiece and the tools working on it. In every system the accuracy of the relative positions of the various machined surfaces is often not satisfactory.
Another system described in U.S. Pat. No. 6,183,404 of K. Deufel employs a tool comprised of a drivable spindle and a cutter or grinder couplable to the spindle. The spindle is poked through a small-diameter hole in a hollow workpiece and an automated gripper can engage underneath the workpiece to fit the cutter/grinder to the spindle so that an interior surface of the workpiece can be finished. Such a special-duty system has only limited applicability and requires that the workpiece be shifted from station to station, in each of which it is held by a separate gripper, in order to work on other surfaces or holes in the workpiece.